Thin Mix HMA, Bonded Friction Course, and Open Graded Friction Courses (OGFC) (also known as Porous friction Courses, Gap Graded Asphalts, or Stone Matrix Asphalts) have grown in popularity in recent years due to their important advantages over the roadways paved with dense graded hot mix asphalt (HMA) and over concrete pavements. These advantages include a reduced risk of hydroplaning, improved drainage, improved visibility, coarse surface for improved friction values, and reduced noise. However, the disadvantages of OGFC have been well documented and include reduced durability, debonding of the OGFC layer, and stripping in the OGFC and underlying areas.
The durability issues have been largely addressed by the use of a Bonded Friction Course (BFC) system that employs the use of improved polymerized tack coat materials and processes. One such Bonded Friction Course system, known as the NOVACHIP process and described in U.S. Pat. No. 5,069,578, uses a specialized “Spray Paver” machine to apply a thick layer of polymer modified tack coat immediately before a thin gap-graded HMA layer is applied. This polymer modified tack coat wicks into the new gap graded mix by displacement and water vaporization. The tack coat provides a degree of adhesion or bonding between the layers and also acts to reduce slippage and sliding of the layers relative to other layers in the pavement structure during use or due to wear and weathering of the pavement structure. The thick application of the tack coat further seals minor cracks in the existing surface layer and forms a strong bond between the new HMA layer and the existing pavement.
However, the NOVACHIP bonded friction course system can be prohibitively expensive due to the requirement that the specialized “Spray Paver” machine be used. Currently, each spray paver machine costs almost $500,000.00, and many contractors and state agencies cannot justify the expense. However, without the use of the NOVACHIP Spray Paver, the thick layer of emulsified polymer modified tack coat used in a bonded friction course system would be very difficult to work with. The thick layer of emulsion tack coat would have a very slow cure rate, resulting in unacceptable delays and also tracking of the tack coat layer. Tracking occurs when the tack or bonding coat is picked up on the tires or tracks of vehicles traveling over the coated surface. Where this occurs, the asphalt compositions often are tracked onto other pavement surfaces causing disruption to the surrounding area. This tracking also reduces the effectiveness of the tack coat by displacing a portion of the intended volume from the area awaiting a new pavement layer.
Insufficient adhesion between a new layer of pavement and an existing base course, a previously laid pavement layer, or a prepared pavement surface can cause pavement separation and cracking during construction of the structure, as well as subsequent failures and premature deterioration of the pavement structure and/or surface. Such conditions often require costly repairs, can cause damage to vehicles traveling on the surface and may cause dangerous traffic conditions threatening damage to property and injury to vehicles and passengers.